Fluorosulfuric acid (IUPAC name: sulfurofluoridic acid) is the inorganic compound with the chemical formula HSO₃F. It is one of the strongest acids commercially available. The formula HSO₃F emphasizes its relationship to sulfuric acid, H₂SO₄; HSO₃F is a tetrahedral molecule. It is a colourless liquid although commercial samples are often yellow.